Personally I would get a used Dell Optiplex / Lenovo ThinkCentre / HP EliteDesk. Either the "mini" / "micro" if you really want small. I am a fan of the SFF (Small Form Factor) size and the Dell Optiplex models. In particular the 70XX versions that offer Intel AMT / vPro for remote management.

I run a Proxmox server on my Dell Optiplex SFF 7070. With the SFF size you can add two low-profile PCIe cards, if desired. In my case I added a 10Gbit NIC.

The reason I would go with a used Dell / Lenovo / HP is that these tend to be well-built machines that run for years and years 24/7 in offices and proven to work well.

Or a used tower form factor PowerEdge if you want a little bigger, but still designed to be quiet in an office environment.